The present invention relates to a process for producing a semiconductor device, and more specifically to a process for producing a semiconductor device especially well adapted to formation of well regions of CMOS or the like with a high precision.
As is well known, the well region of CMOS has heretofore been formed by ion implantation of p- and n-type impurities into the surface of a semiconductor substrate with an acceleration energy of about 50 to 100 KeV, followed by a heat treatment to diffuse the above-mentioned impurities into the depths of the above-mentioned semiconductor substrate.
The above-mentioned conventional process has involved various problems such as a necessity for a high-temperature and lengthy heat treatment and a difficulty in providing a high-precision well region.
In view of this, there is a proposal of forming a well region with a very high ion acceleration voltage in ion implantation processing. For example, a thesis under the title of "MeV Implantation for Silicon Device Fabrication" in Solid State Technology, May (1984), includes a description pointing out that formation of a well of CMOS by ion implantation with a high energy is not only able to reduce the cost increase accompanying a heat treatment and the difficulties caused by deformation of a wafer or the like as compared with conventional processes, but also is effective in preventing soft errors caused by incidence of .alpha. rays since a high concentration region can be formed in the deep portion of a semiconductor substrate. According to high energy ion implantation, however, a difficulty is encountered in implanting ions in a predetermined region with a high fidelity to a mask having a fine pattern necessary for formation of ULSI (ultra-large scale integrated circuit) because of a large dispersion in the flight of ions. Besides, in high energy ion implantation, the controllability of the active impurity concentration in the surface region of the substrate in which a MOSFET is to be formed is so poor that a difficulty may be encountered in precisely adjusting the impurity concentration in the surface region to a desired value. Thus, various problems are presented in putting the process into practical use. Furthermore, a thick film made of a heavy element must be employed as the mask in high energy ion implantation, so that not only materials usable in the mask are limited, but also no simple process like selective oxidation that has heretofore be employed can be employed for formation of p- and n-type wells by the self-alignment technique, leading to inevitable adoption of a complicated process for formation of a mask.